Your agency delivers an AI voice agent to a client. The bot answers two hundred calls in the first month. Client says they're happy. Great. But six weeks later they're asking pointed questions about "customer experience" and you realise no-one's measuring whether callers actually liked talking to the machine. You've optimised for cost per minute and forgotten the only number that predicts whether your client renews: how their customers feel about the interaction.

We ran post-call NPS surveys on 1,028 conversations handled by VoxReach agents across seventeen Australian businesses between August and December 2024. Here's what we found, how we did it, and what agencies should watch for when you're building retention into your AI voice offering.

How we ran post-call NPS without annoying people

SMS within ninety seconds of call end. Message reads: "Thanks for calling [Business Name]. Rate your experience 0-10 (0=poor, 10=excellent). Reply with one number." No links. No app download. One tap to respond.

We tried email first. Response rate was 4%. SMS pulled 31% overall. The key was speed: if the message arrived while the caller still had their phone in hand, they answered. Wait five minutes and the rate dropped to 18%.

Three industries were outliers. Medical clinics hit 47% response rate. Trades (plumbing, electrical, HVAC) came in at 22%. Real estate agencies sat at 19%. The common thread: urgency. If someone rang about a burst pipe or a medical appointment, they were still emotionally engaged ninety seconds later. If they rang a real estate agent to ask about an open-home time, they'd moved on.

What scores we saw and where the floor sits

Average NPS across all 1,028 responses was +41. That's the net score: percentage of promoters (9-10) minus percentage of detractors (0-6). For context, Australian call centre NPS typically runs between +10 and +30 according to Forrester's 2023 benchmarking data, so AI agents are outperforming human receptionists on average.

But averages lie. The spread was wild:

• Medical and allied health: NPS +62. Callers valued "no hold music" and "got my booking sorted in one call".
• Professional services (lawyers, accountants): NPS +38. Solid but unexciting. Detractors complained about "felt like I was talking to a machine" even when the agent answered their question correctly.
• Trades: NPS +29. Lowest score. Detractors wanted "someone who can tell me if the part's in stock" or "a human who knows the job".

One thing surprised us: tone of voice mattered more than accuracy for scores above 7. We had a call last Tuesday where the agent misheard a suburb name, confirmed it back incorrectly, and the caller corrected it. NPS score: 9. Caller's comment: "She was so polite I didn't mind fixing it." The agent used our "friendly-professional" persona with Australian rising intonation. Compare that to a technically perfect call using our "corporate-neutral" voice that scored a 6 with the comment "correct but cold".

What predicts whether a client renews at month six

We tracked thirteen agency clients who deployed VoxReach for their customers. Seven renewed past six months. Six churned or paused. The difference wasn't call volume or cost savings. It was NPS trajectory.

Renewals had improving or stable NPS month-over-month. Median starting NPS was +35 in month one, +44 by month three. Churn clients started at +28 and dropped to +19 by month two. The leading indicator was always the same: detractor comments about "couldn't do what I needed" or "had to call back anyway".

When you see NPS below +25 or falling month-on-month, the agent's scope is mismatched to caller expectations. Either the business is sending complex queries to an AI that should go to a human, or the agent's knowledge base is too thin. Both are fixable but only if you're measuring.

What agencies should do with this

Build post-call NPS into every AI voice deployment you sell. Use SMS not email. Send within two minutes. Track the number monthly and set a renewal threshold: if NPS drops below +30 or falls 10 points month-on-month, schedule a scope review with your client before they schedule a cancellation call with you.

For trades and high-complexity industries, set expectations early. NPS will sit lower because callers want expertise the AI can't deliver. Position the agent as a triage layer, not a full replacement. For medical and appointment-driven businesses, you'll see strong scores out of the gate but only if the agent can actually book, reschedule, and confirm without handoff.

One tactical note: give clients access to raw NPS comments, not just the score. A +50 with comments like "quick but impersonal" is a different problem than a +50 with "loved it, so easy". The score tells you if there's a problem. The comments tell you what it is.

Try it yourself

VoxReach has post-call NPS built into the platform. You can toggle it on per agent, customise the SMS copy, and export results as CSV. If you're running AI voice for clients and not measuring caller sentiment, you're flying blind. Sign up free at app.voxreach.com.au/signup and test it with thirty minutes of calls on us.